It is known to vent an airbag provided for use in a motor vehicle to protect an occupant of the vehicle in the event that an accident should occur, especially if the airbag is of the type intended to provide protection in the event that a front impact should occur.
An airbag that is intended to provide protection in the case of a front impact is generally located so that, when inflated, the airbag is positioned in front of the occupant to be protected. Such airbags are typically provided within the dashboards or steering wheels of motor vehicles to protect the occupants from impact with the hard structures behind the airbag.
During a front impact, the motor vehicle decelerates suddenly. Due to inertia, the occupant of the vehicle tends to continue moving, and thus the occupant of the vehicle is, in effect, moving forwardly relative to the rest of the vehicle. The function of the airbag is to decelerate the occupant, preferably in such a way that the occupant suffers no injury. The airbag must be inflated very swiftly and thus, typically, contains relatively high pressure gas. If the airbag were not vented, then because of the gas pressure within it, the airbag would not decelerate the occupant gently, but instead would stop the occupant extremely swiftly and hence may injure the occupant. Thus, many airbags are provided with vent-holes formed in them so that, when the airbag is struck by the occupant, gas can escape from the airbag through the vent-hole or holes, so that the airbag serves the function of decelerating the occupant more gently, to avoid such injury.
In many cases, it has been found to be desirable to control the degree of venting. For example, in a low load accident situation, such as a slow speed accident or an accident in which the occupant to be protected by the airbag is of relatively low mass, then a relatively high degree of venting is appropriate to safely decelerate the occupant. However, in a high load accident, for example a high speed accident, or an accident in which the seat occupant has a relatively high mass, and hence high inertia, a lower degree of venting is often desirable in order to prevent the seat occupant “striking through” the airbag and suffering injury from impact with dashboard or steering wheel structures located behind the airbag.